This article discusses the proposal for an EU emissions trading system for transport and buildings (the ETS2), again with the EU as an international pioneer. How did this controversial proposal come to be adopted in 2023? This study links up to recent EU studies highlighting co-ordination and interaction between the EU institutions. The Commission helped bring about the ETS2 by strategically using different types of issue linking, both positively and more coercively. In the Parliament, senior members of the European Parliament managed to increase support for the ETS2 by inter-institutional networking and achieving concessions as to flexible mechanisms. Amongst the member states, heavyweight ETS2 proponents had help from the Commission in bringing sceptical countries on board. Hence, several types of synergistic interaction can be noted, facilitated also by trilogues as the more intimate arena for final compromises. Not least, the increasing role of policy packaging highlights the need for further research on changing inter-institutional dynamics.